Highly absorbent articles such as disposable diapers, adult incontinence pads and briefs, and catamenial products such as sanitary napkins, are the subject of substantial commercial interest. Highly desired characteristics of such products are thinness and sustained fit. For example, thin diapers fit better under clothing, provide more freedom of movement to the wearer, and are less noticeable than bulky diapers.
The ability to provide thinner absorbent articles such as diapers has been contingent on the ability to develop relatively thin absorbent cores or structures that can sequester large quantities of discharged body liquids, in particular urine. Upon the discharge of a gush of body fluid into an absorbent article, three distinct mechanisms for handling the fluid may be considered; viz. acquisition, distribution, and storage. Cores in absorbent articles often comprise layers or members to perform these functions. A member may perform one or more functions. For example, a member comprising cellulose fibers and superabsorbent particles may perform fluid distribution and storage as well as some fluid acquisition.
Fluid distribution within an absorbent article is important because absorption of relatively large quantities of fluid in the discharge region of an absorbent article (i.e., the crotch region) tends to increase the bulkiness and weight of this region considerably. This bulk between the legs leads to decreased comfort for the wearer. Further, the localized weight in the discharge region, together with the relatively large dimensional changes as fluids are absorbed, can cause the crotch of the article to sag. This, in turn, can lead to gaps between any leg cuffs or leakage barriers and the skin of the wearer, especially in the discharge region. This can facilitate leakage of fluid from the product, particularly upon subsequent gushes of fluid. Transport or distribution of liquid from the discharge region to the rear and/or front of the article ameliorates localization of the bulk and weight of the fluid. If the need to provide substantial storage capacity in the crotch region is reduced, the resulting design flexibility can enable products which provide enhanced sustained fit and comfort, and can reduce the incidence of leakage.
Fluid distribution from the discharge region to the back and/or front of the article often requires transport against gravity. For example, transport of fluid from the crotch area to the waist area of a diaper with the wearer in a standing position can require the fluid to rise up to 20 cm against gravity, depending on the size and design of the diaper. This distribution of fluid against gravity is typically accomplished via a wicking mechanism. More specifically, the fluid is transported by means of capillary forces within a porous matrix. Typically, the porous matrix comprises cellulose fibers, although other fibers or porous media (e.g., foams) are known in the art.
In addition to wicking the fluid against gravity, the capillary forces in the distribution member must be sufficient to dewater any fluid acquisition materials to the desired level of residual moisture. The level of residual moisture in the regions of the absorbent core closest to the wearer's skin tend to have a significant effect on the dryness of the skin, and/or the wearer's perception of how well the article absorbs fluid.
The rate at which fluid is transported from the discharge region to other regions of an absorbent article is also of critical importance. The propensity for free fluid to leak from an absorbent article is reduced if the fluid is transported to the location where it will be stored (i.e., typically absorbed osmotically by a superabsorbent polymer) as rapidly as possible. A relatively high rate of fluid transport for a given cross-sectional area of the member (i.e., flux) allows the absorbent article comprising the member to have a narrow crotch region while achieving the desired levels of performance in terms of leakage and skin dryness. A narrow crotch facilitates good fit and wearer comfort.
Hydrophilic porous structures with relatively small pores tend to facilitate high capillary forces which enable aqueous fluids to be wicked to relatively high heights against gravity. However, small pores also tend to diminish the permeability of the structure to fluid, so the rate of fluid transport through the structure is impaired. A tradeoff between wicking height and wicking rate typically exists for a given class of porous materials with specific surface properties and fiber (or strut) dimensions.
Amongst other factors, surface tension of the fluid is important in governing the capillary forces responsible for fluid distribution within a porous structure such as an absorbent article. The surface tension of the fluid can be lowered by any surfactants present in the absorbent member. For example, some of the fibrous and foam structures of the art are treated with surfactants to enhance their hydrophilicity. Such surfactants can reduce the surface tension of absorbed fluids undesirably. Capillary forces tend to be lower with fluids having relatively low surface tension. Fluids with low surface tension can be also difficult to desorb from porous structures (e.g., acquisition members). Furthermore, fluids with low surface tension tend to wet surfaces such as skin more readily than fluids with relatively high surface tension, and tend to be more prone to leak through small gaps or openings such as pinholes or pores in the backsheet or cuffs of an absorbent article. Thus, it is desirable for the distribution member (and other components) to lower the surface tension of the fluid as little as possible, and preferably not at all.
Another desired property of the fluid distribution member is its ability to maintain adequate fluid distribution and wicking rate when subjected to forces typically encountered during normal use of an absorbent article. In order to wick aqueous fluids effectively, it is necessary for the material comprising the porous matrix of the distribution member to be hydrophilic. Hydrophilic porous materials conventionally used in diaper cores (e.g., cellulosic fibers) tend to absorb some water when wetted with an aqueous fluid. This absorbed moisture tends to plasticize the material, thereby softening or weakening it, making is susceptible to collapse under the mechanical and gravitational forces exerted on the member during normal use of the absorbent article. Collapse of the structure can lead to undesirable effects, including changes in the pore size, capillary forces and wicking properties of the member. The presence of any superabsorbent particles within the structure also can have an adverse effect on the fluid distribution capability of the member. Superabsorbent particles tend to swell and soften significantly as they absorb fluid. This can disrupt the structure and impede the flow of fluid through the member.
Although it is advantageous for the distribution member to resist collapse during use, it is desirable for it to remain non-rigid, i.e., relatively flexible and soft. This allows the absorbent article comprising the member to conform to the body of the wearer, thus providing good fit and comfort.
Yet another desired attribute of the distribution member is resilience and/or resistance to stress relaxation. Absorbent articles are often subjected to some level of compression during packaging in order to facilitate a relatively compact package. This reduces shipping, handling and storage costs relating to the finished product, and enhances consumer convenience. The forces applied to the articles in order to compress them for packaging may also cause the distribution member within the articles to become compressed. Upon removal of an absorbent article from the package, it is desirable for the acquisition member to re-expand to its original dimensions in order to provide the intended fluid handling properties.
It is further desirable that the materials comprising the fluid distribution member should be non-hazardous, non-irritating, free from malodors, and preferably white in color.
Accordingly, it would be desirable to provide a fluid distribution member for an absorbent article, where the fluid distribution member has the following attributes:                1) Capillary forces sufficient to distribute fluid throughout the member and to dewater any acquisition members to the desired degree during use of the absorbent article        2) Fluid distribution at a rate sufficient to prevent leakage        3) Does not lower the surface tension of fluid excessively        4) Resilient and resistant to stress relaxation        5) Soft and flexible        6) Non-hazardous and non-irritating        7) Free from malodors        8) Preferably white in color Surprisingly, it has been found that distribution members comprising certain non-biopersistent inorganic vitreous microfibers are capable of having all of these attributes.        
Inorganic vitreous microfibers have been disclosed as being useful in disposable absorbent products for a variety of purposes. However, none meet all of the above desired attributes for a distribution member. For example, U.S. Pat. No. 3,525,338 discloses the use of certain elements made up of glass microfibers for fluid storage where the glass microfibers have an average diameter of less than about 0.75 microns. U.S. Pat. No. 4,748,977, discloses certain hygienic products comprising a layer of interlaced mineral fibers whose specific surface area is at least 0.25 m2/g and whose average diameter is under 5 microns. U.S. Pat. No. 6,590,136 discloses certain storage absorbent members comprising glass microfibers. However, the art has failed to recognize the need for such fibers used in disposable absorbent articles to be non-biopersistent.
Distribution members made from materials such as foams have also been disclosed. For example, U.S. Pat. No. 6,013,589 discloses certain foams derived from high internal phase emulsions. However, none of the prior art materials meet all of the above desired attributes for a distribution member.